The primary objective of the Colorado Adoption Project--a longitudinal, prospective adoption study funded by NICHD from 1977 through 1988--is to assess genetic and environmental influences on individual differences in behavioral development among children. Biological parents who relinquished their children for adoption at birth, parents who adopted these children, matched "control" (nonadoptive) parents, and the children (probands and younger siblings) in these families are studied. Adults were administered a 3-hour battery of behavioral tests that included measures of the major factors of cognitive abilities and personality; information pertaining to family background, common medical and behavioral problems, interests and talents, and frequently used drugs have also been obtained. For the children, the assessments employ standard tests to cognitive and language development, personality/temperament, motor development, and health. Environmental assessments in the adoptive and control homes are emphasized. The adopted and control probands and their younger siblings are studied in their homes at 1, 2, 3, and 4; in the laboratory at 7 and 12; and via telephone interviews at 9, 10, and 11. The proposed continuation of HD-10333 will initiate the culmination of the Colorado Adoption Project: The adopted and control children will be interviewed at 13, 14, and 15 years of age. At 16 years of age, they will be administered the same test battery that their parents completed over a decade and a half earlier. During years 12-16 of HD-10333, the numbers of adopted and control children to be studied at 13, 14, 15 and 16 years, respectively, are 367, 261, 185, and 85. Of the various methods available to study the origins of individual differences in behavioral development, the full adoption design employed in the Colorado Adoption Project is the most powerful. The proposed continuation of HD-10333, which will make it possible to reap the final harvest from this landmark study, will substantially advance basic research in child development and is likely to have important implications for child-rearing practices, education, and mental health.